In box 6. of the EIC I see "12" marked on the box where it asks if a child lived with you for half or most of 2018 in the USA mark "12 " if he/ she was alive during 2018

Yes, that's all it is, it's nothing for you to be concerned about.

On the actual IRS form, there's the instruction that "If the child was born or died in 2018 and your home was the child’s home for more than half the time he or she was alive during 2018, enter “12.”